1. Field of the Invention
The present invention relates to a method of detecting a position of an operation line when starting tracking of the operation line using a robot/sensor system in which an operation line on an object is detected by a sensor so as to perform a predetermined operation by moving a tool along the detected operation line, and also relates a robot/sensor system with such detecting function. The present invention is applicable to a robot/laser sensor system for performing various operations such as arc welding, deburring and sealing.
2. Description of Related Art
A technique of operating a robot to move along an operation line such as a weld line while sensing a position of the operation line in advance using a laser sensor mounted on the robot is applied to various robot operations including a welding operation as a real-time tracking technique, the robot having a tool such as an arc welding torch attached to a distal end thereof. This technique enables the robot to move along an actual operation line even when individual objects of operation (for example, fillet welding workpieces) are not positioned precisely. Thus, the technique is widely used as means for improving operation accuracy.
FIG. 1 is a schematic perspective view showing an example of an arrangement of a robot/laser sensor system performing an arc welding on a corner portion WD formed by fillet welding workpieces W1 and W2. As shown in FIG. 1, a welding torch 2 as an operation tool and a laser sensor (a body thereof) 10 are attached to a robot 1 (only a distal portion thereof is shown). The laser sensor 10 scans for sensing surfaces of the workpieces W1 and W2 with a laser beam 4. In the following description, it is supposed that a point 3 representing the position of the robot agrees with an distal end of the tool.
Sign Q0 indicates an initial position of the robot 1, P1 a welding start point which is taught, and P2 a welding end point which is taught. In general cases where positioning of the objects of operation, for example, such as the workpieces W1, W2 to be jointed by welding is not so precise, an actual start point Q1 of the corner portion 4 does not necessarily agree with the taught welding start point P1.
When the robot 1 starts regenerative operation, the robot starts to move the tool tip 3 from the initial position Q0 to the taught point P1, and the laser sensor 10 is turned on to start sensing. Around the time when the tool tip 3 reaches the taught point P1, the laser beam 4 starts to scan the surfaces of the workpieces W1 and W2 to be welded in such a manner as crossing the weld line (corner line) WD to thereby detect the position of the point Q1 on the weld line WD.
Then, after correcting its position to the point Q1, the robot 1 moves along the weld line WD by known real-time tracking. The torch 2 is ignited at an appropriately determined point of time, and welding is performed from the point Q1 to the point Q2. Also when applied to operation other than welding, the robot is guided to the operation start point Q1 by a similar manner of approach, and starts to move along the operation line (a deburring line, a sealing line and the like).
In order that the operation as described above can be started and performed smoothly, it is necessary that positioning of the workpieces W1 and W2 to be welded (objects of operation) be performed with relatively high accuracy (relatively small error) with respect to the range of scanning by the laser beam 4. Since the laser sensor 10 is primarily a sensor for narrow-range sensing, the range of scanning by the laser beam 4 cannot be considered a large one. Therefore, if the positioning of the workpieces W1 and W2 to be welded (objects of operation) is not accurate adequately, there is a possibility that the weld line (operation line) WD cannot be detected.
The most popular method which has been conventionally adopted to deal with this problem is outputting an alarm signal to stop operation if an operation line is not detected within a predetermined time, or before the robot travels a predetermined distance. Such method, however, is one of the causes of drop of operation efficiency.
Another method proposed is designed so that the robot is made to retry its operation when an operation line cannot be detected. The problem is that such method is burdensome since it needs preparation of complicated programs.
Further, in conventional methods, the probability of false detection is high, if an obstacle F (external to the objects of operation) or an obstructively-shaped portion G (belonging to the objects of operation) which hinders normal detection of a position of the operation line lies in the vicinity of the operation start point Q1 and if the objects of operation W1 and W2 are erroneously positioned, causing the taught point P1 to become closer than a normal position to the obstructively-shaped portion G.